The invention relates to a method for the cylindrical grinding of a one-piece workpiece, the contour of which is defined by a continuous longitudinal axis and, in addition to a first longitudinal region, which is cylindrical with respect to said longitudinal axis, also has a second longitudinal region, in which the radial distribution of mass in relation to the longitudinal axis is non-uniform.
Workpieces of this kind are known. They are contoured in accordance with a continuous longitudinal axis, this longitudinal axis simultaneously being a center line and an axis of rotation during subsequent operation. However, only some of them have one or more longitudinal portions of cylindrical cross section which are rotationally symmetrical in relation to the longitudinal axis. In another longitudinal region, the radial distribution of mass is non-uniform because the radial circumferential contour is eccentric or not rotationally symmetrical in some other way with respect to the longitudinal axis. The best-known example of such workpieces are the balancer shafts in modern engines, especially those for motor vehicles. The increasing use of such balancer shafts is the product of the mutually contradictory demands for smoothness from these engines, for low fuel consumption data and for lightweight construction in general. However, the use of balancer shafts is not restricted only to motor vehicle engines but also extends to compressors and other technical areas.
In the specialist jargon of those in the industry, such workpieces are referred to as “unbalanced”. This means that a workpiece of this kind rotating alone is associated with problems of unbalance since the rotary motion is non-uniform and is disturbed by vibrations or wobbling movements. With the increasing use of balancer shafts and similar workpieces, there arose the demand for high-precision grinding of said workpieces, at least in the cylindrical and rotationally symmetrical longitudinal regions thereof, in an economical production process, despite the unbalance behavior thereof.
There have already been various reflections on how this demand could be met with the known means of grinding technology. The knowledge of the applicant on the subject is made up of its own operational practice, from analyses of in-house tests and from discussions among professionals of the kind which customarily take place at specialist conferences, exhibitions and similar occasions. There is no known documentation or publication relating thereto, however.
Thus, consideration was given to producing said workpieces selectively with a considerable allowance in the second longitudinal region thereof in such a way that an approximation to rotational symmetry and hence smooth concentric running would be expected. After grinding, the excess allowance would have had to be removed. However, such a grinding method would be not only very involved and expensive but would also entail a reduction in quality. This is because removal of material by turning or milling after grinding, the latter being a fine machining process, would lead to distortion of the workpiece, making it impossible to comply with the required dimensional and shape tolerances.
The idea of grinding these difficult workpieces by mounting between centers had to be abandoned. It would be expected that grinding said workpieces between centers would be possible only with considerable outlay owing to their instability and the workpiece geometry. For example, an axial contact pressure of the kind which generally arises when grinding between centers would have led to deformation precisely of the weak, eccentric second longitudinal region.
Finally, the tried and tested method for centerless cylindrical grinding was also considered. In this case, however, the experience hitherto has been almost exclusively with completely rotationally symmetrical workpieces. It was therefore known that a relatively severe unbalance of the workpieces made this grinding process very difficult or even impracticable. During centerless cylindrical grinding, an “unbalanced” workpiece will rotate nonuniformly, that is to say will not allow a uniform rotary motion to take place. This means, first of all, an inaccurate grinding result. It was even necessary to accept that the nonuniform rotary motion would even hinder the driving of the workpiece by the regulating wheel, not even allowing the rotary driving of the workpiece to come about. As is known, conditions in the grinding gap are so difficult that the regulating wheel can only transmit a sufficient torque to the workpiece if the latter is by and large also rotationally symmetrical with respect to the distribution of mass. However, if the drive is not reliable for the process from the outset, centerless cylindrical grinding cannot even be considered for these workpieces.